Hi ,

The category_id is not stored in the res_partner table.

category_id = fields.Many2many('res.partner.category', column1='partner_id',
 column2='category_id', string='Tags', default=_default_category)

Since it's a many2many field , it is stored in a relational table ,

Odoo will create a new relational table linking the two models res_partner and res_partner_category. The table will have a name like this.

 'res_partner_res_partner_category_rel'

select p.id,p.name,rel.category_id from res_partner p left join res_partner_res_partner_category_rel rel on p.id = rel.partner_id

Hope it Helps ,

Kiran K